Boost converters have been widely used in various power conversion applications employing both three-phase and single-phase AC input voltages. The conventional single-switch boost converter has become the most popular topology for single-phase power factor correction. For an RMS AC input voltage in the range of 85 volts to 265 volts, a boost technology can provide a sinusoidal input current and an output DC voltage near 400 volts. However, if the RMS AC input voltage is greater than approximately 265 volts, the DC output voltage for a conventional boost converter has to be increased.
As the DC output voltage increases, the voltage stress on the switching devices in both the boost converter and the following load converter increases. This condition requires higher blocking voltage switching devices. The cost of the higher blocking voltage switching devices is greater than the lower voltage rated switching devices. Additionally, the higher voltage rated devices exhibit higher forward conduction voltage drops and higher switching losses than the lower voltage rated devices, which makes them more energy-dissipative, and therefore less efficient.
An approach for dealing with this situation is to adopt the buck+boost topology, which allows the DC output voltage to be lower than the instantaneous AC input voltage. The DC output voltage may then be maintained at 400 volts or less even though the peak AC voltage is greater. There are several disadvantages to the buck+boost converter, however. First, a large pulsating input current requires a large electromagnetic interference (EMI) filter to counteract its negative effects. Also, the buck switch is subjected to both high voltage and high current stresses. Finally, a large number of silicon devices are typically required to process the power.
For three-phase rectification, the split-boost converter is a very efficient topology that allows the DC output voltage to be less than the peak AC input voltage. The split-boost converter provides two equal output voltages and requires two separate loads. A basic requirement of this topology is that the instantaneous rectified AC input voltage must be in a range that is greater than the individual DC output voltages but less than twice the individual DC output voltages for the converter to function properly. As a result, the conventional split-boost topology may not be used in single-phase, high power factor AC input voltage applications.
Accordingly, what is needed in the art is a way to employ the split-boost topology for AC input voltages that are less than the individual DC output voltage.